Systems utilizing high energy radiation, for example X-radiation and gamma radiation, to examine the internal structure of a solid object are well known to the art. Such systems typically irradiate an object under examination with a high energy beam and utilize detection apparatus to measure the intensity of the radiation that is transmitted through the object.
For example, it is known to the art, especially for medical applications, to use a film to record an image of the X-rays that are passed through a human body. Such a film typically includes a top screen of fluorescent material that fluoresces to produce visible light radiation in response to incident high energy X-rays. The light radiation from the top screen passes to a photosensitive film that reacts to the emitted visible light to physically record an image. Such films are used to provide a radiograph of the irradiated body, the radiograph having a resolution of the order of 5 line pairs per millimeter.
The thickness of the top fluorescent screen determines both the resolution of the radiograph and the X-ray stopping power of the film. The stopping power of the film increases as the thickness of the fluorescent screen is increased, since a thicker screen is better able to interact with incident X-rays and generate corresponding visible light. However, as the thickness of the screen increases, the resolution of the film decreases, since the thicker screen tends to increase the scattering of the visible light that is applied to the photosensitive film.
Although X-ray film produces a radiograph having a relatively high resolution, the film necessarily requires a substantial amount of time to develop and, in addition, the film requires a relatively high level of exposure of X-rays to produce a satisfactory radiograph. Also, the film image is not in a form that readily lends itself to computer storage or analysis.
Accordingly, systems have been developed for more rapidly recording the intensity of X-rays or other high energy rays that are transmitted through a target object. Such systems typically employ a scintillation crystal to convert incident X-rays to corresponding visible light radiation. A photodetector is then used to generate an electrical signal corresponding to the intensity of the visible light. The electrical signal from the photodetector may be readily converted to a digital representation and stored in a memory device or electronically displayed, for example, on a cathode ray tube. Of course, the digital data that is derived from the detector signals is suitable for use with a computer.
Prior art radiant energy imaging systems have employed a scintillation crystal and associated solid state optical detectors, for example silicon photodiode arrays, to generate electrical signals corresponding to the intensity of incident X-rays. Such electronic detection apparatus has been used in conjunction with scanning pencil beams or fan beams of radiant energy to quickly provide a radiograph of a scanned target object at relatively low radiation levels. For example, the MICRO-DOSE.RTM. system, as disclosed in U.S. Pat. No. 3,780,291, employs a scanning pencil beam of radiation and an associated scintillation crystal and photodetector to provide both digital radiation intensity data and a corresponding image of an irradiated target object.
It has been proposed for Computerized Axial Tomography scanning systems to use a fan beam of radiation to irradiate a transverse line on a target body and to illuminate corresponding radiation detectors with the fan beam radiation that emerges from the body. The fan beam in such a system is rotated around the body to scan a particular cross-sectional slice of the body and the radiation detectors are electronically scanned to generate an image of the irradiated slice.
Prior art electronic radiation detection devices have been used to produce electronic radiographic images much more quickly than can be done with film and at lower radiation doses than are required to produce images on an X-ray film. However, the radiographic images produced with such prior art electronic radiation detectors have not had the high resolution that is characteristic of radiographic images produced on film. Therefore, electronic imaging systems have not heretofore been suitable for producing high resolution radiographic images.
More particularly, the thickness of the scintillation crystal in prior art electronic radiation detectors has caused a significant loss of resolution due to the normal spread and scattering of the visible light that is generated within the crystal. In such prior art devices, the photodetector is placed behind an associated scintillation crystal and the penetrating X-radiation is applied to illuminate the forward face of the crystal. Thus, the thickness of the scintillation crystal determines the radiation stopping power of the crystal and also affects the resolution of the visible light that is measured by the photodetector. Therefore, in prior art radiation detection devices, the thickness of the scintillation crystal, while providing adequate radiation stopping power, significantly reduces the resolution of the detector.
Accordingly, it is an object of the invention to provide an effective means to both increase the radiation stopping power of an electronic radiation detector and to increase the associated resolution of the detector.
A further object of the invention is to provide such a high resolution detector that will operate at lower levels of radiation than are required to expose an X-ray film.
Another object of the invention is to provide an improved radiant energy imaging apparatus utilizing the high resolution detector of the invention to produce radiographic images having a resolution of at least 5 line pairs per millimeter.
These and other objects of this invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawings.